Im not sure if this is actually allowed in real football but i feel like it is not.

After this safety (https://rundfined.myfootballnow.com/watch/532#95216) I took the lead and the other team ended up doing an onside kick. Can this be fixed?

link now should work

Perfectly legal. An "onside's kick" is NO different than a "Free Kick". A Free Kick occurs after a safety.

Not a bug.

No fixing required.

This actually happened in the NFL recently - http://www.nfl.com/videos/nfl-game-highlights/0ap3000000450815/Lions-pick-up-onside-free-kick

ya but it's a punt and the receiving team can fair catch it without getting hit if they don't chunder it like green bay did. On mfn you have it where the kicking team lines up like a normal kickoff and kicks a normal onside kick when it's not that. If you put it where the punter punts a sky ball to try and get the receiving team to fumble that would be fine.

Actually, after a safety the kicking team has to put the ball in play by a "free kick". Which can be either a punt or a kick off (I seem to remember that's without a tee).
In the NFL they are used to punt because the return yardage is less. But it's legal to kick the ball too

Either way both a scrimmage kick & a free kick CAN be fair caught in real life football. I'm not sure if a Fair Catch exists in MFN on a free kick, or indeed if a **** of a fair catch exists.

I don't understand this because wouldn't a kickoff with someone holding be way further than a punt? And also, in this case the ball was on the tee because no one was holding it which would be illegal if you're right on the rules.

Sounds like it is ok generally but there might need to be some fixing to have it actually follow the rules.

First, don't take my words as truth. I'm no NFL refree! ;)
Anyhow, kicking is a sector where NFL rules change every so often....

In this case, there may not have been a tee and the ball was placed in a hole on the ground, like rugby used to do. Or, using a hump of sand (current rugby setup). Whatever.

I believe NFL prefers the punt because it allows for a better coverage due to a longer hang time. Pro teams are scared of big returns. So if the kick off cannot result in a touchback (because the KO is not on the 35y line due to a penalty) they will try their hardest to make a non returnable kick. A punt makes a fair catch so much more likely